Real Estate Investments (Development Activities) (Details) $ in Thousands		6 Months Ended
		Jun. 30, 2015 USD ($) development_project	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Aggregate purchase price		$ 41,100
Investment to date		83,104	$ 77,450
Total commitment		$ 17,553,130	$ 18,292,560
2015 Acquisitions [Member]
Business Acquisition [Line Items]
Number of development projects completed | development_project		19
Aggregate purchase price		$ 10,200
Number of remaining projects to be completed | development_project		18
Estimated completion period for remaining projects		12 months
Number of build-to-suit, redevelopment and expansion projects in process | development_project		6
Development projects in progress | development_project		24
Investment to date		$ 67,911
Estimated cost to complete	[1]	26,195
Total Investment	[2]	94,106
Total commitment		12,900
Tenant Improvements		$ 10,700

[1] The Company is contractually committed to fund a developer $12.9 million to complete the remaining 18 build-to-suit developments.

[2] Excludes tenant improvement costs incurred in accordance with existing leases. As of June 30, 2015, $10.7 million of tenant improvement costs were included in land and construction in progress in the consolidated financial statements.